The invention relates to a process for purifying a reducing useful gas by pressure swing adsorption using a so-called ejector operated by a pressurised gas and generating a vacuum which serves to improve the desorption efficiency of the pressure swing adsorption unit, in which the invention uses steam as pressurised gas generated by the heating device in the production of the useful gas. The invention also relates to a device with an ejector operated by steam, the vacuum of which is used to increase the desorption efficiency of the pressure swing adsorption unit.
The production of reducing gases is a process frequently used in technical applications. An example for the production of a reducing gas is the steam reforming process. By heating a hydrocarbonaceous feed mixture with steam, it is, for instance, possible to produce synthesis gas consisting of a mixture of hydrogen, carbon monoxide and carbon dioxide. Synthesis gas is frequently used as fuel gas or feedstock for chemical syntheses. Another frequently produced reducing gas is carbon monoxide.
Most applications require a reducing gas which is free of accompanying gases. A frequently occurring accompanying gas is carbon monoxide which is obtained from the incineration or reforming processes. If the carbon monoxide from the synthesis gas is converted in the so-called CO conversion in the presence of steam, the result is a mixture of hydrogen and carbon dioxide. The latter can be removed from the synthesis gas by various processes. When the carbon dioxide is removed, pure hydrogen is obtained which is an essential feedstock in many syntheses and applications. Other frequently occurring accompanying or foreign gases or foreign gas components which are undesired constituents of reducing gases and need to be separated are residual gas components such as methane and nitrogen.
Many applications include gas-scrubbing processes for the separation of undesired accompanying gases, which are suited for the mentioned applications. Also frequently used are the so-called pressure swing adsorption units in which the gas to be separated undergoes adsorption processes. Pressure swing adsorption units are capable of removing foreign gases almost completely, which can be achieved by gas-scrubbing processes on certain conditions only.
Pressure swing adsorption units use the adsorptive capacity of adsorbing materials. Examples of frequently used adsorbing materials are activated carbon, molecular sieves, silica gel or zeolithes. For gas purification purposes, the adsorbing materials are packed into suitable vessels and the gas to be purified is passed through these vessels under pressure. The foreign gas components are thus adsorbed and the purified useful gas is obtained. After a certain time, the adsorptive capacity of the adsorbing material will be exhausted. The vessel is depressurised and flushed with useful gas so that the adsorbing material releases the adsorbed foreign gas. Subsequently the foreign gas is discharged for further processing or removal. The adsorption and desorption processes are performed in cyclic alternation so that it is possible to ensure a continuous purification process of the supplied useful gas.
In order to further improve the efficiency of the adsorption process, the desorption process can be supported by applying a vacuum after pressure relief. This will allow almost complete removal of the adsorbed foreign gas from the adsorbing material. Subsequently it will be possible to re-use all of the adsorbing material in the next adsorption process. To avoid that vacuum has to be generated by an expensive compressor or other machine, a so-called ejector is used. This ejector uses the pressure of the discharged gas to generate a vacuum by utilizing the Venturi effect. By controlling the valves in a suitable way to ensure that the discharged gas flows through the ejector, it is possible to achieve adequate evacuation of the pressure swing adsorption vessels.
EP 015413 B1 describes a pressure swing adsorption process which includes an arrangement of pressure swing adsorption units which, in cyclic alternation, remove a specific component or several gas components by selective adsorption from a gas mixture. A vacuum generated by means of an ejector is used to desorb the adsorbed foreign gases step by step. The ejector is operated by the pressure of the gases discharged from the pressure swing units, allowing the use of both the discharged useful gas and the discharged desorbed foreign gas if the pressure swing adsorption units and the valves are connected in an appropriate way.
EP 1101731 A1 describes a process for the production of synthesis gas in connection with a pressure swing adsorption unit by which the synthesis gas is freed from the entrained carbon dioxide. The vacuum for the desorption of the pressure swing unit is generated by an ejector which uses the high pressure of the fuel gas when being supplied. The gas leaving the ejector may then be re-used as fuel gas. By a system of feed vessels, both the fuel gas and the vacuum can be adapted to the capacity required.
The vacuum generated by the ejectors, however, is frequently inadequate. For this reason it is necessary to install devices used to further increase the vacuum. This is expensive and in most cases associated with a demand for additional equipment. Apart from this, the pressure of the discharged gas is not always high enough to operate the ejector in such a way that an adequate vacuum is generated. The use of compressed gas is inappropriate for reasons of cost. It is therefore necessary to find a low-cost solution for operating the ejectors at higher efficiency. It is therefore the aim of the present invention to make a gas available which is under high pressure, reasonably priced and, as far as possible, generated in the useful-gas production process and which allows to operate the ejectors in a cost-effective manner.
EP 864351 A2 describes a process for implementing a vacuum swing absorption, in which air is subdivided into an oxygen-enriched and an oxygen-depleted flow, and the vacuum for the vacuum swing absorption is generated by steam under pressure, and the pressurized steam is generated by an exothermic chemical reaction which is fed with the oxygen-enriched flow and in which the oxygen constitutes a reactant. The generation of the vacuum by the steam is carried out by at least one so-called eductor. The process is not suited for the separation of useful gases from a gas mixture as the separated oxygen-enriched gas is used for the generation of the steam. Furthermore, the separated gases are not under pressure.